Our invention relates to an apparatus and a method for molding precisely shaped articles, which require precisely parallel, opposite molded surfaces. More particularly, this invention pertains to a method and an apparatus for parallel aligning of opposing mold surfaces for use in pressing glass or glass-ceramic articles with parallel opposing surfaces.
Articles, such as glass magnetic memory disk substrates for example, must be formed with two surfaces, top and bottom, both substantially flat and parallel. In a process that forms the substrate by pressing a measured volume of molten glass between two tools, i.e., a receiving and opposing portion, it is difficult to produce a part where the sides are exactly parallel. Major sources of this error are: fabrication tolerances in the machine and tools; unbalanced forces on the machine and tools which cause uneven physical deflection; and unbalanced heating of the machine and tools which cause uneven thermal expansion.
All rational effort is made to make the machine with parallel tool mounting surfaces, to place the measured volume of glass on the molding surfaces such that the physical deflection is symmetrical, and to minimize unbalanced heating of the machine. Regardless of this effort, however, parallel tool surfaces are not always attained. For example, in the typical glass pressing, environment thermal distortion is caused by unstable temperature conditions in the pressing room. One flank of either the press's receiving or opposing portion,s alignment ring faces the hot orifice, while the opposite flank faces the cool air in the press room. This temperature imbalance affects the expansion rate of the alignment ring pads located on the receiving and opposing portions. The pads which face the hot orifice expand and lengthen at a higher rate than the pads facing away. When the thermally distorted pads on the receiving and opposing alignment rings touch, the gap between the receiving and opposing portion surfaces will no longer be parallel. Any parts formed while thermal distortion is present will exhibit a glass aberration called wedge.
The present invention is directed toward correcting this prior art problem of wedge. This invention uses the phenomenon of one of these sources of error to compensate for the errors of the others, namely; tc control the unbalanced heating of the machine in a manner to use thermal expansion to correct for the other sources of error. This invention uses preferential heating and/or cooling of the machine to introduce thermal expansions in the opposite direction of the uncompensated error sources to make the tool surfaces parallel during forming of the substrate.